1. Technical Field
The present invention relates, in general, to a method and system for providing waveform representations of heart function, such as those produced by the electrocardiograph. In particular, the present invention relates to a method and system for providing waveform representations of heart function, such as those produced by the electrocardiograph, by synthesizing a standard 12-lead electrocardiograph from a subset of electrodes utilized to derive the standard 12-lead electrocardiograph.
2. Description of Related Art
The present invention presents a method and system for synthesizing a standard 12-lead electrocardiograph from a subset of electrodes utilized to derive the standard 12-lead electrocardiograph. In order to understand why such synthesis is useful, it is helpful to have a basic understanding of the "gold standard" 12-lead electrocardiograph. Accordingly, as an aid to understanding the electrocardiograph, the discussion below presents a brief description of (1) the electrochemical and mechanical operation of the heart, (2) how the electrochemical operation of the heart is transduced into electrical energy which is then used by the electrocardiograph to graphically denote the mechanical operation of the heart, and (3) how the certain specific electrical signals (or "leads") are derived from the electrocardiograph.
The mechanical events of the heart are preceded and initiated by the electrochemical activity of the heart (i.e., the propagation of the action potential). There is a device which transforms the electrochemical activity of the heart into a form visible to the human eye: the electrocardiograph, which produces a visual representation of the electrochemical activity of the heart. The visual representation is known as the electrocardiogram (EKG).
During an EKG, electrodes are attached to the body surface. The electrodes are specially treated to allow the charge carriers within the electrodes (electrons) to communicate with the charge carriers within the body (ions) via electrochemical exchange. Attaching electrodes to the body surface allows the voltage changes within the body to be recorded after adequate amplification of the signal. A galvanometer within the EKG machine is used as a recording device. Galvanometers record potential differences between two electrodes. The EKG is merely the recording of differences in voltage between two electrodes on the body surface as a function of time, and is usually recorded on a strip chart. When the heart is at rest, diastole, the cardiac cells are polarized and no charge movement is taking place. Consequently, the galvanometers of the EKG do not record any deflection. However, when the heart begins to propagate an action potential, the galvanometer will deflect since an electrode underneath which depolarization has occurred will record a potential difference from a region on the body under which the heart has not yet depolarized.
A complete heart cycle is known as a heartbeat. On an EKG, a normal heartbeat has a distinctive signal. Initially, the galvanometer notes a relatively short duration rounded positive deflection (known as the P wave), which is caused by atrial depolarization. Subsequent to this, there is a small but sharp negative deflection (known as the Q wave). Next, there is a very large and sharp positive deflection (known as the R wave), after which there is a sharp and large negative deflection (known as the S wave). When these waves are taken together, they are known as the QRS complex. The QRS complex is caused by ventricular depolarization. Subsequent to the QRS complex, is a relatively long duration rounded positive deflection (known as the T wave), which is caused by ventricular repolarization.
The EKG, in practice, uses many sets of electrodes. But these electrodes are so arranged on the surface of the body such that the signals received will have the similar shape as that just described. Well-known bipolar pairs of electrodes are typically located on a patient's right arm (RA), left arm (LA), right leg (RL) (commonly used as a reference), and left leg (LL). Unipolar electrodes referenced properly are referred to as V leads and are positioned anatomically on a patient's chest according to an established convention (labeled as follows as Leads V1-V6). In heart monitoring and diagnosis, the voltage differential appearing between two such electrodes or between one electrode and the average of a group of other electrodes represents a particular perspective of the heart's electrical activity and is generally referred to as the EKG. Particular combinations of electrodes are called leads. For example, the leads which may be employed in a "gold standard" 12-lead electrocardiogram system are:
Lead I=(LA-RA)
Lead II=(LL-RA)
Lead IIII=(LL-LA)
Lead aVR=RA-(LA+LL)/2
Lead aVL=LA-(RA+LL)/2
Lead aVF=LL-(LA+RA)/2
Lead V1=V1-(LA+RA+LL)/3
Lead V2=V2-(LA+RA+LL)/3
Lead V3=V3-(LA+RA+LL)/3
Lead V4=V4-(LA+RA+LL)/3
Lead V5=V5-(LA+RA+LL)/3
Lead V6=V6-(LA+RA+LL)/3
Thus, although the term "lead" would appear to indicate a physical wire, in electrocardiography the term actually means the electrical signal taken from a certain electrode arrangement as illustrated above.
Furthermore, it will be understood by those within the art that there are certain instances (e.g., when monitoring neonate patients or patients with right-side infarct) where some electrocardiographic electrodes are placed on the right side of the chest. For example, it is well known within the art that right side variations of 12-lead electrocardiographic leads V3, V4, and V5 exist and are referred to within the art as V3R, V4R, and V5R. It is to be understood that well-known variations, such as the examples cited, are to fall within the rubric of the 12-lead electrocardiograph and its attendant electrode placements (and subsets of the 12-lead electrocardiograph and its attendant electrode placements) as such is used herein.
Over the years, health care professionals have built up a body of knowledge wherein they have learned to correlate variations in the EKG with different diseases and heart defects. Formally, this process of correlating is known as "electrocardiography."
Electrocardiography, as practiced by human cardiologists, is primarily a visually oriented art in that the human cardiologists visually inspects a waveform tracing of electrocardiographic measurements taken over time, and on the basis of the morphological (i.e., shape) changes of the waveform over time the human cardiologist makes a diagnosis of heart function. In making such diagnosis, it is essential that the human cardiologist have an accurate characterization of waveform representation, derived from the electrocardiographic measurements, of heart function in that inaccuracies in the waveform will give rise to inaccuracies in diagnosis.
The requirement for an accurate characterization of waveform representation is even more critical for mechanized electrocardiography. That is, machines have been created which have automated many of the functions traditionally performed by human cardiologists.
As has been alluded to, the "gold standard" for accurate characterization of waveform representation of heart function is the 12-lead electrocardiograph described above. Indeed, many of the diagnoses and techniques, which have been developed over the years, are dependent upon the presence of all 12 leads of the "gold standard" electrocardiograph.
However, as is apparent from the above discussion, the "gold standard" 12-lead electrocardiograph requires the accurate preparation and placement of 10 electrodes to derive the 12 leads. Furthermore, each electrode has associated with it a wire which connects the electrode to the electrocardiograph proper. Consequently, obtaining the "gold standard" electrocardiograph requires considerable time and precision associated with accurately preparing and placing the 10 electrodes; furthermore, even after the electrodes are correctly prepared and placed, significant clutter exists arising from the wires and connectors associated with each electrode.
The foregoing problems associated with electrode preparation, placement, and wire clutter have been recognized. Consequently, attempts have been made to "synthesize" the "gold standard" 12-lead electrocardiograph utilizing less than the 10 electrodes ordinarily required to produce the 12-lead electrocardiograph.
Three of the better known attempts to synthesize the 12-lead electrocardiograph have been to synthesize the 12-lead electrocardiograph utilizing the EASI lead system, to synthesize the 12-lead electrocardiogram utilizing the Frank vectorcardiograph lead system, and to synthesize the 12-lead electrocardiogram utilizing a patient-specific 12-lead transformation (invented by Julie Scherer and John Nicklas). Each of these systems will now be discussed in turn.
In the EASI lead system, an attempt is made to derive the full 12-lead EKG from a non-standard 5-electrode lead placement (denoted as the EASI lead system). The 12-lead electrocardiogram is derived from non-standard 5-electrode lead placement via the use of a patient-independent transformation for deriving the 12-lead EKG from the EASI leads. This patient independent transformation was produced via the cross-correlation of twenty-seven simultaneously acquired 12-lead EKGs and EASI EKGs. G. E. Dower, Method and Apparatus for Sensing and Analyzing Electrical Activity in the Human Heart, U.S. Pat. No. 4,850,370 (1989) (hereby incorporated in its entirety). The reason the non-standard 5-electrode lead placement was used was because Dower was trying to devise the best electrode placement which would yield essentially linearly independent signals from which the 12-lead EKG could be derived. As has been emphasized by the foregoing italicization and underlining, all 12 leads provided by the EASI lead system, including all the limb leads, are derived, which means that none of the leads corresponding to any of the leads in the "gold standard" 12-lead electrocardiogram can be acquired directly by use of the EASI system. Furthermore, it is to be understood that because the EASI synthesis requires the presence of all leads formed by the EASI electrodes attached to a patient, the loss of even one electrode will disable the EASI synthesis process.
Given the fact that all leads in the EASI system are derived, the question naturally arises as to how accurate such derived leads are, when compared with actual 12-lead electrocardiograph simultaneously obtained from the same patient. At present, the only known method for making such assessment is the simultaneous application of the EASI 5 electrode set, and the 12-lead EKG 10 electrode set, to the same patient.
In the Frank system, instead of viewing the activity of the heart as a potential difference plotted against a time base as in the electrocardiogram, the activity of heart is viewed as a "spinning" or "rotating" "vector" of varying magnitude located within the three dimensional space of the chest cavity. In the Frank system, the idea is that the "tip" of the vector is intended to be indicative of the motion of that action potential as it spreads throughout the heart. Thus, the idea is that a cardiologist can get a feel for what is occurring within the heart by watching how the action potential is flowing through the heart as is indicated by the Frank "vector." G. E. Dower, Method and Apparatus for Sensing and Analyzing Electrical Activity in the Human Heart, Columns 2-3, U.S. Pat. No. 4,850,370 (1989) (discussing the Frank vectorcardiograph).
The leads constructed from the electrode set utilized to produce the Frank "vectorcardiograph" are nearly orthogonal. Consequently, they produce nearly linearly independent leads, or signals, and thus (as was discussed above in relation to the EASI leads) the Frank leads are interpreted to form good candidates for synthesis of the 12-lead electrocardiograph. However, since the Frank system electrode placement is again different from the 12-lead electrocardiograph lead placement, there is no way to track the accuracy of the synthesis derived from the Frank system except to simultaneously attach and run a 12-lead electrocardiograph in the fashion as was described in relation to testing the accuracy of the EASI system. Furthermore, it is to be understood that because the Frank synthesis requires the presence of all leads formed by the Frank electrodes attached to a patient, the loss of even one electrode will disable the synthesis process.
In the patient-specific 12-lead transformation, a patient-specific 5-electrode system (using a standard 5-electrode lead system, seven leads can be obtained, including all six limb leads (I, II, III, aVR, aVL, and aVF) and a chest lead (V)) is utilized to synthesize a 12-lead electrocardiogram. As the name of the attempt indicates, the synthesis is achieved via the use of a patient-specific transformation. J. Nicklas and J. Scherer, Method And Apparatus for Synthesizing Leads of An Electrocardiogram, U.S. Pat. No. 5,058,598 (Oct. 22, 1991). The patient-specific transformation is achieved as follows. For each patient a conventional full 12-lead EKGs is first taken. From this 12-lead data, a patient-specific transformation which can be used to synthesize a 12-lead EKG from 3 semi-orthogonal leads (such as I, II, and V2) is identified or created via the use of near-real-time numerically and computationally intensive data processing.
It has been found that the baseline performance of this patient-specific transformation can be further improved by segmenting the synthesized EKG waveforms into PR, QRS and ST segments. It has been shown that by adaptively segmenting the EKG, typically 12 to 24 segments, the error rate on reconstruction can be further reduced. However, this segmentation to achieve the gain in accuracy has resulted in a significant increase in processing requirements in obtaining the patient-specific transformation.
There are significant shortcomings with respect to the foregoing described attempts to synthesize the 12-lead EKG from a less than 10 electrode set, a few of which will be detailed here. Both EASI and Frank lead syntheses have at least three significant shortcomings, present in all of their incarnations: (1) both the EASI and Frank systems utilize a fixed, non-standard, lead system, which requires technicians to learn and be familiar with lead systems beyond those normally associated with the 12-lead EKG and its standard subsets (which is definitely a potential source of human error, since accuracy of lead placement has been shown to be difficult for a large percentage of nurses, Drew, Ida and Sparacino, Accuracy of Bedside Electrocardiographic Monitoring: A report on Current Practices of Critical Care Nurses, Heart & Lung 1991; Vol. 20, No 6, 597-609); (2) because in both the EASI and Frank syntheses all leads are derived there is no easy way to check the accuracy of the derived leads; and (3) both the EASI and Frank syntheses require the presence of all their leads to effect synthesis. With respect to lead placement, as can be seen by the description of the 12-lead system, above, the number and positioning of electrode placements makes the complexity of the 12-lead system high, making it difficult to learn; thus, the EASI and Frank requirements of learning new lead systems and electrode placements add more complexity and a likelihood of an increase in human error. With respect to accessing the accuracy of the derived leads, it was discussed above that the only way to check either the EASI or Frank 12-lead syntheses is to actually connect a separate 12-lead system simultaneous with the EASI or Frank systems to check for accuracy. And, with respect to the fact that both the EASI and Frank syntheses require the presence of all leads, the EASI and Frank systems are not robust in that the loss of even one EASI or Frank electrode will disable both the EASI on Frank syntheses.
The patient-specific transformation system, in all its different incarnations, has at least five significant shortcomings that will be appreciated by those within the art: (1) the system requires the presence, and previous use of, a 12-lead electrocardiograph in order to gather data from which a patient-specific transformation can be produced; (2) the system, even in its most basic and stripped-down incarnation, is computationally and numerically intensive, with such computational and numerical intensiveness increasing as more accurate versions of the system are implemented; (3) the system requires that a base set of leads be chosen and then that a transformation be calculated for such chosen base set, and consequently requires the recalculation of a patient-specific transformation every time clinical requirements force the selection of a new base set; (4) the accuracy of the patient-specific transformation is directly proportional to the duration of which the standard 12-lead electrocardiograph is attached to the patient; and (5) no mention is made as to how the accuracy of leads synthesized can be verified or checked.
Thus, EASI and Frank syntheses require the use of fixed lead sets utilizing electrode placement not compatible with the standard 12-lead electrocardiograph, while the patient-specific transformation requires that a 12-lead electrocardiograph be measured and then utilized to compute a transformation for a particular lead set for a particular patient. The foregoing gives rise to a number of difficulties.
With respect to EASI and Frank systems, the non-standard lead sets increase the amount of training necessary for personal and make it difficult to check the EASI/Frank syntheses for accuracy. With respect to the patient-specific transformation, the requirement that a 12-lead electrocardiograph be taken increases the time and effort necessary to eventually construct the synthesis; furthermore, in many cases, such obtainment of the 12-lead EKG baseline data is not practicable, such as cases where a patient has a surgical wound at the location where one would need to apply the electrode.
There have been attempts in the past to synthesize one or more leads of a 12-lead electrocardiograph using fewer than the standard number of ten electrodes. However, these systems have all produced unacceptable results. Nicklas at column 2, lines 35-54.
With respect to the foregoing, it is apparent that a need exists for a method and system which will allow the following: (1) synthesis, by use of patient-independent transformations, of one or more leads of the 12-lead electrocardiograph from one or more subsets of the 12-lead electrocardiograph electrode placements and wherein such synthesis produces user acceptable results; (2) a way of showing the accuracy of the synthesis as a whole and/or synthesis leads based on the particular subset of the 12-lead electrocardiograph electrodes in use; (3) a quick and easy real-time check on the accuracy of the one or more synthesized leads; (4) the quick and easy addition or removal of 12-lead electrocardiographic electrodes from the initial subset used, and subsequent synthesis, by use of a transformation, of one or more leads of the 12-lead electrocardiograph on the basis of the resulting subset of electrodes and leads derived therefrom; and (5) optionally, provide all foregoing described synthesis operations but with the use transformations which, although still patient-independent, are geared to a particular patient-profile.